Grindstone



FIG. 1 is a perspective view of a grindstone showing my new design;

FIG. 2 is a front end view thereof;

FIG. 3 is a rear end view thereof;

FIG. 4 is a left side view thereof;

FIG. 5 is a right side view thereof;

FIG. 6 is a top plan view thereof; and,

FIG. 7 is a bottom plan view thereof.

The broken line showing is for illustrative purposes only and forms no part of the claimed design. 

The ornamental design for a grindstone, as shown and described. 